Semiconductor wafers are tested prior to singulation into individual die, to assess the electrical characteristics of the integrated circuits contained on each die. A typical wafer-level test system includes a wafer prober for handling and positioning the wafers, a tester for generating test signals, a probe card for making temporary electrical connections with the wafer, and a prober interface board to route signals from the tester pin electronics to the probe card.
The test signals can include specific combinations of voltages and currents transmitted through the pin electronics channels of the tester to the probe interface board, to the probe card, and then to one or more devices under test on the wafer. During the test procedure response signals such as voltage, current and frequency can be analyzed and compared by the tester to required values. The integrated circuits that do not meet specification can be marked or mapped in software. Following testing, defective circuits can be repaired by actuating fuses to inactivate the defective circuitry and substitute redundant circuitry.
Different types of probe cards have been developed for probe testing semiconductor wafers. The most common type of probe card includes elongated needle probes configured to electrically engage die contacts, such as bond pads, or other contacts on the wafer. An exemplary probe card having needle probes is described in U.S. Pat. No. 4,563,640 to Hasegawa et al.
Although widely used, needle probe cards are difficult to maintain and unsuitable for high parallelism applications, in which multiple dice must be tested at the same time. In addition, needle probe cards are not suitable for some applications in which the dice have high count die contact requirements, such as bond pads in dense grid arrays. In particular, the long needles and variations in the needles lengths makes it difficult to apply a constant gram force to each die contact. Long needles can also generate parasitic signals at high speeds (e.g., &gt;500 MHZ).
A similar type of probe card includes buckle beams adapted to flex upon contact with the wafer. This type of probe card is described in U.S. Pat. No. 4,027,935 to Byrnes et al. Although better for high count die contacts, and high parallelism applications, buckle beam probe cards are expensive, and difficult to maintain.
Another type of probe card, referred to as a "membrane probe card", includes a membrane formed of a thin and flexible dielectric material such as polyimide. An exemplary membrane probe card is described in U.S. Pat. No. 4,918,383 to Huff et al. With membrane probe cards, contact bumps are formed on the membrane in electrical communication with conductive traces, typically formed of copper.
One disadvantage of membrane contact bumps is that large vertical "overdrive" forces are required to penetrate oxide layers and make a reliable electrical connection with the die contacts on the dice. These forces can damage the die contacts and the dice. In addition, membrane probe cards can be repeatedly stressed by the forces, causing the membrane to lose its resiliency. Use of high probe temperatures can also cause the membrane to lose resiliency.
Another disadvantage of membrane probe cards is the CTE (coefficient of thermal expansion) mismatch between the membrane probe card and wafer. In the future, with decreasing size of each die contact, higher parallelism requirements, and increased probing temperatures, maintaining electrical contact with the die contacts will be increasingly more difficult. In addition, because of relatively large differences between the CTE of membrane probe cards and silicon wafers, maintaining electrical contact between a large number of dice and a membrane probe card will be almost impossible.
Yet another limitation of conventional test systems, and a disadvantage of conventional probe cards, is that full functionality testing must be performed at the die level rather than at the wafer level. These tests require a large number of connections with the dice, and separate input/output paths between the dice and test circuitry. For functional test procedures on dice having multiple inputs and outputs, an input/output path must be provided to several die contacts at the same time. The number of dice that can be tested in parallel is always limited by the number of drive only, and input/output channels the tester provides, as well as the die contact arrangements on the dice. The number of drive only and input/output channels is fixed for a particular test system by its manufacturer.
To maintain speed characteristics for high count die contacts, the die contacts must be distributed throughout, or around the edges of the dice in a dense array. With this arrangement it is very difficult to parallel probe multiple dice using needle type probe cards, and impossible with dice having high count die contacts. Buckle beam probe cards are a costly alternative for probing dice having high count die contacts.
In view of the foregoing, improved probe cards capable of testing wafers with large numbers of dice, and high count die contacts, at high speeds, are needed in the art. In addition, probe cards capable of expanding tester resources to accommodate high parallelism, and high count die contact testing applications are needed in the art.